1. Field of the Invention
This invention relates from a smooth rotation of an object by an electric drive motor to a desired position. More specifically, the invention relates to control of a galvanometer used to rotate a diffraction grating in a spectrophotometer.
2. Description of the Prior Art
A typical prior art system for rotating an object, such as a diffraction grating in a spectrophotometer, is shown in FIG. 1. The purpose of the system 10 is to rotate the diffraction grating 11 to a precise angular position so that the grating 11 diffracts incident light which is coming from a source (not shown) to a destination (not shown). The grating 11 must be at a precise angle so that the light diffracted to the source is at a predetermined wavelength. The grating 11 is rotated to various positions by the system so as to diffract various wavelengths. It is important that the grating 11 be quickly rotated to each of its positions, so that the spectrophotometer can utilize in quick succession the various diffracted wavelengths.
The prior art system 10 is a standard analog feedback servo system. A position sensor 12 attached to galvanometer 13 provides a signal proportional to the angular position of output shaft 14 of galvanometer 13 to which grating 11 is attached. The conventional microprocessor 15 (typically an 8 bit microcontroller) provides commands by means of a digital to analog converter 16 to rotate the output shaft 14 of galvanometer 13 to a desired position. The position sensor 12 output is subtracted from the input commands and the resultant "error" signal 17 drives a conventional closed loop control circuit 18, including lead and lag networks, which in turn drive the galvanometer output shaft 14 to the desired position.
A major deficiency of this prior art system is that at high gain, the galvanometer 13 does not respond well to discontinuous input commands, e.g., step functions. Typical responses by the galvanometer 13 to a step include overshoot, which is detrimental to the galvanometer if the swing takes the galvanometer output shaft rotation past the specified limits, and exacerbation of resonances of modes outside of the angular rotation, such as a decaying resonance of the grating about the axis perpendicular to the axis of rotation and parallel to the grating surface.
The rotation of the grating 11 produced typically by the prior art system 10 of FIG. 1 is shown in FIGS. 2A, 2B and 2C. FIG. 2A shows the angular position (vertical axis) versus time (horizontal axis) of the grating 11 of FIG. 1. FIG. 2B shows the velocity (i.e., first derivative of position with respect to time) of the grating versus time. The abrupt "step" 22 is apparent. FIG. 2C shows the acceleration (i.e., second derivative of position with respect to time) of the grating versus time. The abrupt impulses 24 in acceleration are apparent, meaning that the grating 11 of FIG. 1 is "jerked" by a sudden acceleration and again by a sudden deceleration. This jerking is the cause of the aforementioned overshoot and resonances.
One prior art approach to the problem is to electronically filter the commands output by microprocessor 15 (see FIG. 1) so as to smooth out the commands to the galvanometer 13. However, this approach does not eliminate the jerking because typically the velocity of rotation still changes very abruptly, since the velocity achieves maximum value typically in much less than one time constant of the filter, so that resonance (i.e., ringing) still occurs. Also, a filter used for such purposes takes a long time period to settle to its final output level; this long time period slows the overall response of the system.